A laser diode is a semiconductor device which emits laser light as a result of applying a forward current in excess of a threshold value. The physical structure of an edge emitting laser diode typically includes an active layer located between N type and P type layers. The ends of the active area not bounded by the N type and P type layers are each covered by a mirror facet. The laser light is generated in the active layer. The mirror facets at the ends of the active area form an optical resonator. Light generated in the active area which does not propagate parallel to the axis of the optical resonator leaves the active area through the sides of the resonator. Light traveling in the optical resonator parallel to its axis is repeatedly reflected from the mirror facets.
Light is generated in the active area through processes of spontaneous emission and stimulated emission. In the process of spontaneous emission an electron moves from a state in the conduction band to a state in the valence band within the active layer. The energy lost by the electron is converted to a photon. Spontaneous emission occurs so that photons are generated at random in time and propagate in all directions within the active layer. Because of this, spontaneous emission does not generate a coherent beam of laser light. In the process of stimulated emission, a photon transfers its energy to an electron in the conduction band. When the energized electron moves to a lower energy state in the valence band it emits two photons which propagate in the direction of the incident photon.
Without a forward current above the threshold value applied to the laser diode, the distribution of energies of the electrons in the active layer is such that spontaneous emission dominates the light generation process. No light amplification occurs because there is not a sufficient fraction of the population of electrons existing in conduction band energy states to allow stimulated emission to occur to the degree that it will replace those photons lost from the active layer.
However, when a forward current above the threshold value flows through the active layer, the energy states of a large fraction of the electrons are shifted into the conduction band. As a result, stimulated emission dominates the light generation process. Photons which leave the active area are outnumbered by those which are reflected from the mirror facets. The photons reflected from the mirror facets cause light amplification through the stimulated emission process. When stimulated emission dominates the light generation process oscillation occurs in the resonator.
Photons generated by stimulated emission from photons reflected from the mirror facets propagate parallel to the optical axis of the resonator and either cause stimulated emission of additional photons or are reflected from the mirror facets. As a result, generation of light propagating parallel to the optical axis is favored. As the forward current flowing through the active layer increases, the fraction of the population of electrons in the conduction band increases causing a corresponding increase in the generated laser light. The length of the active region between the mirror facets is an integral multiple of the half wavelength of the laser light in the active layer. The oscillation of the laser light in the resonator establishes a standing wave along the length of the active layer. A portion of the generated laser light is transmitted through the mirror facets to form the cone of light output from the laser diode.
For laser diode currents above the threshold value and below the typical maximum operating currents, the output power is approximately linearly related to the magnitude of the drive current. The drive circuits used in the commercial application of laser diodes are typically designed to rapidly switch the laser diode between the condition in which the laser diode is emitting laser light and the condition in which the optical output power is substantially zero. Generally, this is accomplished by steering a current of substantially constant value through the laser diode in response to a drive signal in the asserted state. When the drive signal is not asserted the current is bypassed around the laser diode. The intent of driving the laser diode with a current of substantially constant value is to maintain the optical output power at a substantially constant value over the time in which drive current is flowing through the laser diode.
A vertical cavity surface emitting laser (VCSEL) diode operates by the same physical principles as an edge emitting laser diode using a different physical configuration. In a VCSEL diode, a series of layers of a dielectric material are located above and below the active layer. The dielectric constants of these layers are selected so that a small percentage (in the range of 3%) of the generated light propagating normal to the layers is reflected from the interface between the active layer and the adjacent dielectric layer and between each of the dielectric layers. A portion of this generated light is transmitted through the dielectric layers and is emitted normal to the surface of the VCSEL diode. The volume of the active layer in a VCSEL diode is considerably less than that of a edge emitting diode having comparable optical output power. There are anomalies in the operating characteristics of VCSEL diodes which, for some applications, must be corrected to make the use of the VCSEL diode suitable.